The present invention relates to a fuel electrode for high-temperature solid electrolyte fuel cells and a method of manufacturing the electrode.
Heretofore, nickel/zirconia (Ni/ZrO2) cermets have been employed for fuel electrodes in commercialized high-temperature solid electrolyte fuel cells.
These Ni/ZrO2 cermets are typically produced by the below alternative processes.
(1) A process in which NiO/ZrO2 is sintered and then reduced to provide a Ni/ZrO2 cermet, (e.g. the Ceramatec process described in Proceedings of SOFC-NAGOYA, p.24); and
(2) A process, known as the Westinghouse process, in which ZrO2 in a crude Ni/ZrO2 cermet is caused to grow into spaces between Ni grains by the EVD technique, (Japanese Kokai Patent Publication No. 61-153280).
Aside from the above, porous platinum (Pt) materials obtainable by sintering platinum pastes have been used in ZrO2 sensors and the like or in basic research.
However, the Ni/ZrO2 cermet electrode manufactured by the first-mentioned process is disadvantageous in that when the fuel cell is run over thousands of hours at temperatures near 1,000xc2x0 C., the Ni grains therein are sintered thus degrading the electrode and ultimately causing exfoliation of the electrode. Increasing the proportion of ZrO2 to avoid this sintering detracts from the performance of the electrode.
The Ni/ZrO2 cermet manufactured by the second process is resistant to sintering and insures a satisfactory electrode performance, but, since it involves an EVD step, the overall manufacturing process is complicated and the cost of manufacture is increased.
The porous platinum electrode is disadvantageous in that the platinum reacts with the impure metal in the fuel and is vaporized in a continuous operation in a reducing atmosphere resulting in early aging of the electrode. Furthermore, platinum is an expensive metal and the manufacture of porous platinum electrodes is costly.
The object of the present invention is to overcome the above disadvantages and provide an improved fuel electrode which does not undergo sintering or aging even when the fuel cell is operated continuously for a long time at a high temperature and a simple method of manufacturing the electrode.
The fuel electrode of the invention for high-temperature solid electrolyte fuel cells is characterized in that it is a porous element composed of a high melting point metal having a melting point of not less than 1,900xc2x0 C. or an alloy containing such a high melting point metal.
As species of the high melting point metal with a melting point of not less than 1,900xc2x0 C., there may be mentioned ruthenium (Ru), osmium (Os), rhodium (Rh) and iridium (Ir).
As apparent from the melting points presented below, Ru, Os, Rh and Ir melt at higher temperatures than Ni and Pt and are more resistant to sintering. Because they are highly resistant to sintering, ceramification is not required. Furthermore, since the resistance to sintering is high, it is feasible to design a system with a long interfacial dimension of the three-phase zone of ZrO2/Metal/Gas (if such a structure is designed with Ni, sintering soon occurs) and thereby reduce all the reaction polarization, diffusion polarization and resistance polarization of the electrode.
Of the above-mentioned metals, Ru is the most advantageous in that it has:
(1) a high melting point and stability even in a reducing atmosphere;
(2) a low cost; and
(3) a high catalyst activity for steam reforming of CH4 which is an important factor in electrode performance.
Osmium (Os) is also useful but in view of the high vapor pressure of OSO4 and its high toxicity, it is somewhat disadvantageous for commercial use as compared with Ru.
The fuel electrode of the present invention for high-temperature solid electrolyte fuel cells can be manufactured by dissolving or dispersing a powder of at least one of Ru, Os, Rh and Ir or at least one of ruthenium chloride, osmium chloride, rhodium chloride and iridium chloride in an organic solvent or water, coating an electrode material with the resulting solution or dispersion, sintering the same and finally reducing it. It should be understood that the powdery metal and metal chloride can be used in combination with each other.
The sintering operation is preferably conducted in an oxidizing atmosphere at a temperature of 400 to 1,000xc2x0 C. and the reduction reaction is preferably conducted at a temperature of 500 to 1,300xc2x0 C.
Preferably the reduction is conducted in a reducing atmosphere, particularly in a hydrogen (H2) atmosphere.
The organic solvent to be used is preferably a low-boiling point solvent and more desirably an alcohol of 1 to 6 carbon atoms. The most advantageous alcohols are propanol and butanol.
In terms of the ease of porosity control and handling, it is good practice to employ a high molecular weight compound soluble in the organic solvent or in water. Among such high molecular weight compounds are polybutyl alcohol, polyvinyl alcohol and methylcellulose.
Compared with the conventional Ni/ZrO2 cermet electrode, the electrode according to the present invention has the following meritorious characteristics:
(1) Substantially no sintering occurs even when the fuel cell is run for thousands of hours at 1,000xc2x0 C.; and
(2) Each of the reaction, diffusion and resistance polarization values are as small as several mV at 500 mA/cm2. 
Particularly when Ru is employed, not only an excellent electrode performance is ensured but also the manufacturing cost is kept down resulting in profitable commercial application.
Furthermore, since the manufacturing method according to the present invention is a simple process comprising coating an electrode material with an aqueous or organic solution or dispersion of a powdery high melting point metal and/or a high melting point metal chloride, sintering the same, and then reducing it, the manufacturing process is simple and the cost of manufacture is low. Moreover, the concomitant use of a high molecular weight compound soluble in the organic solvent or water facilitates porosity control and handling.
The following examples and comparative examples are intended to illustrate the invention in further detail and should by no means be construed as defining the metes and bounds of the invention.